Thermosetting resins based on ethylenically unsaturated N,N-bis-imides ##STR1## wherein U represents a divalent radical containing a carbon-to-carbon double bond and A represents a divalent radical having at least two carbon atoms are well known in the art. These bis-imides are conventionally prepared by addition of two moles of an anhydride such as maleic anhydride to one mole of an aromatic diamine such as 4,4'-methylene dianiline. The initial reaction product, a bis(maleamic acid) EQU U (COOH)--CO--NH--A--NH--CO--U (COOH)
is then dehydrated to form the final cyclic N,N-bis-imide. These bis-imides may be converted to polyimide resins by heating, as taught by Grundschober and Sambeth, U.S. Pat. No. 3,380,964. Alternately, they may be coupled with appropriate multifunctional reactants such as diamines having the general formula H.sub.2 N--B--NH.sub.2, wherein B represents a divalent radical having up to 30 carbon atoms--see for example Bargain and Combet, U.S. Pat. No. Re. 29,316. The exceptional chemical resistance and thermal stability of polyimide resins have found them numerous applications, in for example the manufacture of composites, insulating materials, adhesives, and the like.
Inasmuch as the rate of polymerization of the bis-imides can be controlled by temperature, it is customary in the art to carry out a partial polymerization to form an oligomer or "pre-polymer" which still retains some degree of solubility in selected solvents. Pre-polymer solutions are then used to impregnate fibrous materials such as graphite or glass cloth. These impregnated mixtures are conventionally referred to as "pre-pregs", The solvent is then stripped from the "pre-preg" by means of a drying oven. In the course of this operation, some further polymerization of the pre-polymer occurs, increasing its molecular weight. Finally the "pre-preg" is subjected to higher heat and pressure to complete the polymerization and form the final cured composite article.
Whereas the above process has been used for several years, difficulties do exist. Pre-polymers of conventional bis-imides are soluble only in a few high-boiling solvents such as N-methylpyrrolidone and dimethylformamide. It is extremely difficult to eliminate these solvents completely from the intermediate "pre-preg". If traces of solvent remain, the final cured polymer may contain bubbles, cracks, or other flaws which can render it useless for its intended purpose. This problem is particularly serious with printed circuit boards, an important application for polyimide resins, where flaws cannot be tolerated. Furthermore solutions of pre-polymer in these solvents normally have poor storage stability and must usually be used within twenty four hours of preparation.